Cover Crops Improve Water Productivity of Deficit Irrigation and Control Wind Erosion for Semi-Arid Conditions.

Wind erosion may constitute one of the greatest challenges to soil health in semiarid crop production systems. Independent of climate, grass crops grown in some combination with legumes or forbs provide a biomass residue barrier to control wind and water erosion, protect seedlings, and limit evaporation in addition to adding organic matter for improved soil aggregation. Regional climate variability can impose dramatic differences in water availability as illustrated by a ratio of the mean annual precipitation (MAP) to potential evapotranspiration (PET). The MAP:PET ratio of 0.20 – 0.50 determines semiarid sites like Bushland, TX with a ratio of 0.41 and ~500 mm MAP compared to ratios of 0.50 – 0.75 for subhumid sites including Akron, CO and Mandan, ND with ratios 0.61 and 0.75 (resp.) despite their lower average MAP of 427 mm. Other potential cover crop benefits may include retention of nutrients in the root zone and improved drainage of excess soil water at sites in Missouri, Iowa, and Minnesota where MAP normally exceeds PET. Management of cover crops in semiarid regions must use limited precipitation and available soil water to provide soil conservation. A cover crop system for reducing wind erosion is to plant wheat (Triticum aestivum L.) after cotton (Gossypium hirsutum L.) harvest, use fall rain for vegetative growth, and chemically terminate the wheat in the spring before resuming cotton production with conservation tillage. Cover crop residue effectively intercepts raindrop impact to increase infiltration and repartitions evaporation from the soil to transpiration by the crop for increased yield, but greater infiltration and transpiration may not offset the water used for cover crop production. Under semiarid conditions, crop rotations provide residue that can be managed to increase crop water use yield with a diminished hazard of cover or cash crop failure in the absence of rain. Except as necessitated for soil erosion control, irrigation pumping restrictions to conserve groundwater may render the choice between cash or cover crop irrelevant.